Transparent and opaque metal-semiconductor-metal photodetectors

Abstract

An improved metal-semiconductor-metal (MSM) photodiode, specifically a new high responsivity AND high bandwidth photodetector, resulting in a high gain-bandwidth product is disclosed. The disclosed device is an MSM photodiode in which the anode and cathode are made of different materials of differing opacity and possibly including different electrode dimensions as well. Using an opaque anode and a transparent cathode reduces surface reflections off the opaque electrodes allowing more light to be absorbed within the active semiconductor region. However, it concurrently keeps the transit distance for the slower moving holes to a minimum. Thus, the long tail in the impulse response due to hole collection is minimized, resulting in increased bandwidth.

Having thus described the invention as above, We claim: 
     
       1. A photodetector comprising: a semiconductor substrate having an active region on one surface thereof,   a first set of electrodes comprising a first conducting material deposited on the active region of the substrate   a second set of electrodes comprising a second conducting material deposited on the active region of the substrate,   wherein the first set and second set of electrodes are made of different materials having different degrees of opacity.   
     
     
       2. A photodetector as claimed in claim 1, wherein the active region of the substrate is InGaAs, InGaAsP, GaAs, or silicon. 
     
     
       3. A photodetector as claimed in claim 1, wherein the first set of electrodes is negative and more transparent than the second set of electrodes. 
     
     
       4. A photodetector as claimed in claim 3, wherein the negative electrode is a substantially transparent material. 
     
     
       5. A photodetector as claimed in claim 3, wherein the negative electrode is a semi-transparent material. 
     
     
       6. A photodetector as claimed in claim 4, wherein the substantially transparent material is a conducting oxide. 
     
     
       7. A photodetector as claimed in claim 6, wherein the conducting oxide is indium oxide, tin oxide, or cadmium tin oxide. 
     
     
       8. A photodetector as claimed in claim 3, wherein the second set of electrodes is positive. 
     
     
       9. A photodetector as claimed in claim 8, wherein the positive electrode is a metal or silicide. 
     
     
       10. A photodetector as claimed in claim 9, wherein the positive electrode further comprises an additional conducting layer. 
     
     
       11. A photodetector as claimed in claim 10, wherein the additional conducting layer is gold. 
     
     
       12. A photodetector as claimed in claim 1, wherein the first set of electrodes is 50 to 100% transparent. 
     
     
       13. A photodetector as claimed in claim 1, wherein the first set of electrodes is 60 to 100% transparent. 
     
     
       14. A photodetector as claimed in claim 1, wherein the second set of electrodes is from 0-30% transparent. 
     
     
       15. A photodetector as claimed in claim 1, wherein the second set of electrodes is from 0-20% transparent. 
     
     
       16. A photodetector as claimed in claim 8, wherein portions of the negative electrode have a width greater than the corresponding portions of the positive electrode. 
     
     
       17. A photodetector as claimed in claim 8, wherein the width of portions of the negative electrode is from 1 to 50% greater than the width of corresponding protions of the positive electrode. 
     
     
       18. A photodetector as claimed in claim 8, wherein the electrodes are antireflective. 
     
     
       19. A photodetector comprising: a semiconductor substrate selected from the group consisting of GaAs, InP, and silicon, said substrate having an active region,   a barrier enhancement layer on said active region,   a first set of electrodes deposited on the barrier enhancement layer comprising a first conducting material which has a transparency greater than 50%,   a second set of electrodes deposited on the barrier enhancement layer comprising a second conducting material which has a transparency of less than 20%,   an antireflective coating deposited on the top of the photodetector,   wherein the photodetector has a mesa structure.   
     
     
       20. A photodetector as claimed in claim 19, wherein the mesa has sidewalls which are passivated by deposition of a semi insulating material.